Organization, Basis of Presentation and Summary of Significant Accounting Policies (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Organization, Basis of Presentation and Summary of Significant Accounting Policies (Details) [Line Items]
Remaining contractual payments under ground lease agreements	$ 41,770
Taxable income minimum distribution portion not subject to federal taxation (in percentage)	90.00%
Real estate improvements	$ 54,500	$ 41,400
Acquisition transaction costs	4	824	$ 965
Allowance for doubtful accounts receivable	6,400	5,200
Payroll related costs capitalized	$ 1,200	$ 1,200	$ 1,100
Number of segments | segment	1
Minimum | Building
Organization, Basis of Presentation and Summary of Significant Accounting Policies (Details) [Line Items]
PPE useful life (in years)	39 years
Minimum | Building Improvements
Organization, Basis of Presentation and Summary of Significant Accounting Policies (Details) [Line Items]
PPE useful life (in years)	10 years
Minimum | Furniture and Fixtures
Organization, Basis of Presentation and Summary of Significant Accounting Policies (Details) [Line Items]
PPE useful life (in years)	3 years
Maximum | Building
Organization, Basis of Presentation and Summary of Significant Accounting Policies (Details) [Line Items]
PPE useful life (in years)	40 years
Maximum | Building Improvements
Organization, Basis of Presentation and Summary of Significant Accounting Policies (Details) [Line Items]
PPE useful life (in years)	20 years
Maximum | Furniture and Fixtures
Organization, Basis of Presentation and Summary of Significant Accounting Policies (Details) [Line Items]
PPE useful life (in years)	10 years